1. Field of the Invention
The present invention relates generally to dewatering devices, and, more particularly, to an improved active and electro-osmotic dewatering system.
2. Background Description
Many common building and insulating materials have a capillary pore system that can become saturated with water. This is especially the case where the material, such as a building's foundation, is in contact with a particulate moisture source, such as soil. Long-term continuation of this saturated condition is undesirable and may lead to deterioration of the materials. Similarly, sludges, dredged spoils, and fine aggregates can become laden with moisture, making them unstable to stockpile and heavy to transport or dispose.
One traditional procedure for dewatering particulate materials is a combination of heating and ventilation. However, these procedures are very slow and utilize large amounts of energy. Further, as with any process utilizing heat, there is a risk of thermally induced warping and cracking of the structure.
Accumulated water may also be pumped from the particulate material. However, employing this method on any significant volume of particulate requires either a substantial number of pumps or a great deal of suction, as capillary forces in fine-grained materials make it difficult to extract water. Additionally, a simple pumping system is relatively easy to overwhelm with wet weather.
Another known technique for eliminating water from porous or particulate materials is electro-osmosis. The walls of the capillaries in most common building materials are covered with an electrically charged, adsorbed water film, referred to as an electrical double layer. It has been established that, if such a porous body is subjected to an electrical field, part of the double layer will tend to migrate under the influence of the field. Some of the free liquid in the pores is carried along with the double layer, leading to a significant reduction in the moisture content of the porous body or particulate.
There are, however, practical drawbacks to the use of electro-osmotic dewatering systems. First, conventional electro-osmotic systems are rather inefficient, and therefore relatively easily defeated by wet weather. The application of an electrical charge to a wall of porous building material typically involves the use of electrodes provided or installed in the porous material, and connected through the material to a grounding electrode. When the electrodes are polarized, there is a migration of water molecules towards the cathode. However, after the system has been in operation for a time, the electrodes become covered with coherent films of gas formed by electrochemical reactions at the electrode surfaces. These films have a very high electrical resistance, leading to deterioration in the electrical characteristics of the system and lowered system efficiency. A similar problem arises from the fact that the anodes of the system are subject to a high degree of electrolytic corrosion. Where electrodes are installed specifically for dewatering, this corrosion results initially in reduced system efficiency, and, eventually, in complete electrical discontinuity at the electrode. Thus, it becomes necessary to discontinue use of the system or replace the electrodes. Alternatively, where the structure's internal reinforcing steel is used as the anode, heavy corrosion of the electrode is damaging to the structure itself.
Accumulated gases present another problem in existing electro-osmotic dewatering systems. As noted above, any passage of current through water will result in some electrolysis of the water. This can generate hydrogen, oxygen, and chlorine gases that can accumulate in an empty extraction casing. These accumulated gases may also react, producing an undesirable energetic event, such as a fire or explosion.
Extant electro-osmotic dewatering systems also require a great deal of space to install. A typical electro-osmotic dewatering system, such as that taught by U.S. Pat. No. 6,372,109 to Bjerke et al., employs horizontally arranged electrodes and a surface array. These horizontally disposed systems generally use a separate steel casing as an anode and collect water at a cathode/extraction pipe situated away from the area to be dewatered—that is, at least two probes are required to accomplish the dewatering.
Prior art dewatering systems are also subject to encrustation in the extraction pipe. Where the metal is a cathode, the pH at the surface of the pipe rises, allowing precipitation of minerals from the groundwater. This precipitate can obstruct or clog the slots or perforations in the extraction pipe.
Electrical isolation is also a problem with existing electro-osmotic dewatering systems. If the surrounding soil is dewatered to such an extent that the resistance around the metal pipe increases, the dewatering circuit is effectively broken. Anyone touching the metal pipe, or even an extraction pump attached to the system, is a better conductor to ground than the metal pipe, making extant systems rather dangerous, especially for systems that run at high voltages.